This invention relates to a control method and apparatus for an internal combustion engine equipped with a supercharger, and more particularly it is concerned with an improved method and apparatus of controlling supercharged pressure to an optimum level in accordance with the exhaust temperature to thereby prevent thermal rupture of the supercharger and the elements of the exhaust system and reduce fuel consumption.
Generally, in an internal combustion engine equipped with a supercharger, air shows a rise in temperature due to compression of the air by the supercharger for supply to the engine, and in order to achieve the effect of supercharging by raising the supercharged pressured at the low engine speed range, the area of the nozzle for the exhausts to flow into the turbine casing is reduced as much as possible, so that the exhaust pressure rises particularly in the high engine speed range. This tends to cause knocking to occur if ordinary gasoline fuel is used. To avoid this phenomenon, it is usual practice to operate the engine at high speed and under high load by setting the ignition time at a point much delayed than the minimum spark advance for best torque which suits the air and fuel supplied to the combustion chamber. This has resulted in the temperature of the exhausts rising to an inordinately high level and the supercharger and the elements of the exhaust system being exposed to the heat of high temperature, thereby causing the risk of thermal rupture to occur in them. Also, the engine output has been restricted by knocking, and fuel consumption has been adversely affected.
To obviate this problem, proposals have hitherto been made to increase an ignition delay required for initial stage combustion in the combustion process to thereby delay the occurrence of knocking, to use fuel of high ignition delay in place of ordinary gasoline, and to incorporate an additive, such as tetraethyl lead, or fuel (of the benzol or alcohol system) of high antiknocking ability in the ordinary gasoline. Also, to accomplish the same object, programs have been under way to reduce compression ratio to keep the pressure and temperature of the end gas as low as possible in the combustion process, to introduce variable compression ratio, and to use a swirl producing construction for the combustion chamber to increase flame velocity, so as to delay the knocking producing ignition time.
Some disadvantages are associated with the aforesaid proposals. The use of other fuel than ordinary gasoline is not desirable so long as gasoline engines are concerned. Avoidance of air pollution by engine exhausts takes priority over an improvement of engine performance, so that the incorporation of tetraethyl lead in the gasoline should be avoided. The incorporation in ordinary gasoline of antiknocking fuel makes it necessary to use specific equipment, thereby increasing cost.
In view of the situation described hereinabove, it is considered advisable to control the supercharged pressured in an internal combustion engine equipped with a supercharger in such a manner that the supercharged pressure is kept at a predetermined level when the engine operates at high speed and under high load. It is also considered advisable that when the aforesaid control is effected, the air-fuel ratio of the mixture is set at a level at which the mixture is substantially enriched to shift the knock limit to the advance side so that the exhaust temperature limit may not be exceeded, to thereby obtain an exhaust gas temperature drop and improved engine power. In this case, there is the risk that fuel consumption might be adversely affected by setting the air-fuel ratio in a manner to enrich the mixture. Also, depending on how the supercharged pressure is controlled, thermal rupture of the supercharger mounted in the exhaust system and the elements of the exhaust system might result.